This invention relates generally to traction control systems for motor vehicles whereby the tractive forces of the vehicle wheels are automatically controlled so as to prevent wheelspin. More specifically, the invention pertains to a traction control system relying on wheel brakes for traction control and featuring provisions for precluding the possibility of vehicle turnover when the vehicle is running on a curve.
Vehicle traction control in response to the rate of wheelspin is per se not new in the art. Generally, in this type of traction control system, vehicle traction is so controlled as to limit the slip of the drive wheels at a rate (So in FIG. 3 of the drawings attached hereto) where the driving force and lateral force are equally high. The possibility of wheelspin when the vehicle is steered in either direction is thus reduced by assuring the high lateral force without substantially sacrificing the propelling force.
A problem has been encountered in connection with systems relying on wheel brakes for traction control. When the vehicle makes a sharp turn, with the consequent lifting of the inner wheels off the road surface, the conventional traction control system has operated to brake the wheels that are about to spin. Then the differentials included in the vehicle drive line will cause a rapid increase in the engine power transmitted to the outer wheels which are in contact with the road surface. Thus, with its inside wheels braked and its outside wheels subjected to greater driving forces, the vehicle in turns has been exposed to the possibility of rolling over.
U.S. Pat. No. 3,893,535 and Japanese Patent Laid-Open Publication No. 62-87630 represent examples of prior art traction control systems. These prior art systems are similar in that, upon detection of wheelspin, the engine output torque is automatically decreased to overcome the wheelspin. The cited Japanese Patent Laid-Open Publication further teaches to inhibit the reduction of engine power at low vehicle speed in order to prevent the possibility that the vehicle may become incapable of starting up because of the spinning of only one drive wheel. However, both prior art systems are silent on the subject of the possibility of vehicle rollover in turns.
The above noted problem has remained unsolved with traction control system incorporating wheel brakes. The prevention of vehicle turnover requires accurate and quick detection of inside wheel liftoff and the immediate overriding of the normal traction control and engine power control.